1. Field of the Invention
This invention relates to riser management systems. More particularly, this invention relates to a system, an apparatus, and related methods for sensing riser dynamics.
2. Brief Description of the Related Art
A problem presented by offshore hydrocarbon drilling and producing operations conducted from a floating platform or vessel is the need to establish a sealed fluid pathway between each borehole or well at the ocean floor and the work deck of the vessel at the ocean surface. This sealed fluid pathway is typically provided by a drilling riser system. Drilling risers, which are utilized for offshore drilling, extend from the drilling rig to a blowout preventer (BOP) and Lower Marine Riser Package (LMRP), which connect to a subsea wellhead. Production risers extend from a surface vessel to a subsea wellhead system.
The drilling riser, for example, is typically installed directly from a drilling derrick on the platform of the vessel by connecting a series of riser joints connected together. After connecting the riser to the subsea wellhead on the seabed, the riser is tensioned by buoyancy cans or deck mounted tensioner systems. The riser is projected up through an opening referred to as a moon pool in the vessel to working equipment and connections proximate an operational floor on the vessel. In drilling operations, the drill string extends through a drilling riser, the drilling riser serving to protect the drill string and to provide a return pathway outside the drill string for drilling fluids. In producing operations, a production riser is used to provide a pathway for the transmission of oil and gas to the work deck.
Basic components of a riser system typically also include a tensioning system designed to provide lateral load resistance while providing a somewhat constant vertical tension. The tension is normally applied to a tensioning ring attached to the top of the riser and below a telescopic joint. A diverter seals around the drill pipe and diverts gas and drilling returns away from the drill floor. A slip or telescopic joint is designed to decouple the vessel and riser from vertical motions and maintain an integral seal for the riser pipe. A ball or flex joint provides a pinned connection to reduce the transmission of bending moments in the riser caused by a misalignment between the joints. Riser connectors are made up of sections typically bolted together with flanges or threadedly connected, each section being typically from 60-90 feet in length. Each section typically has a central riser pipe that is normally about 18-24 inches in diameter. Buoyancy devices are typically provided to reduce tensioning requirements, mainly in deep water conditions where the top tension required is greater than the available tensioning capacity. Various subsea equipment is located on the seafloor. The subsea equipment associated with a drilling riser might include a flex/ball joint, the BOP/LMRP, the wellhead, and a wellhead conductor. The BOP/LMRP typically includes valves and sensors controlled by a BOP/LMRP controller which is connected to the surface via an umbilical cord which includes a data conductor. The umbilical cord can be positioned between the BOP/LMRP controller or other subsea equipment and a computer or controller remotely positioned on a deployment platform of the vessel. An umbilical spool can, in turn, be positioned on the deployment platform for readily storing and deploying the umbilical cord.
Other more specialized riser equipment includes a fill-up valve designed to prevent collapse of the riser pipe due to the differential pressure between the inside of the riser pipe and the surrounding water, an instrument riser joint typically used to monitor the tension and bending due to environmental conditions which allows for adjustment in top tension and vessel positioning, vortex suppression equipment which help suppress vortex induced vibrations typically found in conditions of high current and long riser length, and an emergency riser release which provides a specialized riser release system to prevent catastrophic failure typically found in conditions where incorrect vessel positioning or extreme environmental conditions may occur.
The riser has design requirements that include operation and/or survival in extreme conditions in both connected and disconnected modes. Deepwater applications, especially, require close attention to the vertical dynamics of the riser. This generally requires an active riser management program. One goal of riser management is to determine the tension/buoyancy requirements and the operating limits based on a combination of the environmental parameters, the vessel capability, the drilling program (for drilling risers), and the operational constraints. Another goal or series of goals for both drilling and production risers is to manage stresses and loading of individual riser sections to provide for fatigue analysis and thus allow the operator to formulate an enhanced inspection, maintenance, and riser section rotation program. The environmental parameters include, among other things, wave height and period, water depth, current, wind, and tides. The vessel capability includes tensioning capacity, physical interface geometry, and vessel motion characteristics in terms of Response Amplitude Operator (RAO). The drilling program includes riser joint configuration, mud weights, and placement of components. The operational constraints to be considered are drilling modes, upper and lower displacements and forces, combined stresses, and tensioner losses.
The normal modes encountered in offshore drilling operations, for example, include normal or drilling mode, suspended or connected and nondrilling, and hangoff or disconnected mode. The drilling mode is that combination of environmental and well conditions in which normal drilling activities can be safely conducted. The connected and nondrilling mode is the mode when only circulating and tripping out drill pipe is conducted. The disconnect mode is when environmental conditions exceed the limits for safe operation in the connected and nondrilling mode and require the riser to be disconnected to prevent possible damage to surface or subsea equipment.
The loading on both drilling and production risers include internal and external hydrostatic pressures generated by the drilling mud and sea water, weights or buoyant forces generated by auxiliary components, and wave and current actions. The hydrodynamic forces generated by the waves can be based on a regular wave or a wave spectrum. The hydrodynamic forces generated by the current are calculated based on Morrison's Equation using the shape, roughness, Reynold's number, Keulegan-Carpenter Number, and orientation of auxiliary equipment. Standard values of drag and inertial coefficients have been developed. Loading on the riser system can additionally be generated by vortex shedding generated by the current, resulting in vortex induced vibration (VIV). VIV can be generated either in-line or cross-flow, and can induce high stresses if the shedding frequency matches the natural frequency of the riser.
Mathematical methods for the solution of the complex loading and motion in the riser are based on static, frequency domain, and time domain solution techniques. The static solution does not take into account any dynamics and is not as accurate for the overall analysis of the riser system, but can provide current and steady state loading information. The frequency domain solution uses linearization techniques to simulate the dynamic portion of the loading and can accurately model the loading, if the dynamics are moderate as compared to the static loading. The time domain can accurately model the dynamic loading and provide the most accurate modeling of both the linear and nonlinear conditions. The time domain solution can encompass a direct integration of the nonlinearities in the calculations, and requires a large number of solution iterations. The advent of more powerful computers has resulted in reasonable solution times and has made the time domain solution the most desirable method of solution.
The operational limits are based on providing a combination of tension, vessel location, and operating mode to maintain ball/flex joint angles, material physical property requirements, system component requirements, and prevent system component failure. Obtaining data to provide to the computer systems, however, has proved more problematic. Especially regarding drilling operations, system integration has been difficult due to the insular nature of the different control systems on the drilling rig. The operator interfaces currently in use have inherent accuracy limitations due to low update rates and do not capitalize on the importance of lower flex joint angle (“LFJA”)/upper flex joint angle (“UFJA”) differential, nor the importance of modeling the dynamic shape of the riser.
Current systems of monitoring ball/flex joint angle values do not provide riser managers sufficient data to properly maintain such operational parameters. Some recent systems include instrument modules that can provide static differential angle of the riser. The systems were, however, originally designed to support drilling operations, not riser management systems, and are not suitable as a basis for riser analysis because they provide only a limited set of measurements, and typically only for the lower flex joint. Current systems generally provide only static accuracy. That is, current systems generally only provide a static lower flex joint angle of inclination, values of which are affected by lateral acceleration, and which does not allow for real-time management of the riser system. Further, the inclination is referenced to a coordinate system separately assigned to the individual instrument housing or case, itself, rather than a globally assigned coordinate system. Thus, such systems are difficult to integrate with other more globally based systems.
In an attempt to acquire data on the behavior of a riser under determined conditions, a more recent French system is being developed which utilizes a series of instrument modules consisting of lateral accelerometers and inclinometers connected along the length of the riser string and to the lower marine riser package to determine the two-dimensional deflected shape of the riser. The modules are connected to a computer through a data transmission cable extending the length of the riser string. This system, however, does not provide dynamic angular position and orientation of the riser. The system also apparently only provides two-dimensional (planar) angular measurements. Further, this system has not been shown to be practical because each module is individually connected to the data transmission cable through individual cable leads along the length of the data transmission cable. Thus, the data transmission cable requires a series of terminators/taps along the length of the cable. If a section of the riser carrying one of the modules is removed, the module will need to be either moved to another section, or the module will need to be disconnected from the data transmission cable and cap added to replace the removed module. In either scenario, the procedure is rather labor-intensive and requires disruption of the drilling operation and/or the management of the riser.